getDiffRanges: better fallback for absent patch #6291

Job	Run time
Packaging: Config file (ubuntu-latest, linked)	1m 11s
Packaging: Config file (macos-latest, linked)	1m 22s
Packaging: Config file (ubuntu-latest, default)	56s
Packaging: Config file (windows-latest, linked)	4m 14s
Packaging: Config file (macos-latest, default)	1m 45s
Packaging: Config file (windows-latest, nightly-latest)	7m 18s
Packaging: Config file (windows-latest, default)	3m 55s
Packaging: Config file (ubuntu-latest, nightly-latest)	1m 5s
Packaging: Config file (macos-latest, nightly-latest)	1m 47s
	23m 33s

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